[sdiy] Dumbest Mistakes with Synths Contest Anyone?

[R. D. Davis]

Quothe Michael Bacich, from writings of Fri, Aug 04, 2006 at 07:59:28PM -0700:
> I think you need to get in there and take a good look at the Juno's  
> output jack board, especially in light of your comment about it not  
> having the bass response you expect.  That symptom sounds familiar --  

Hi, thanks for the reply.  After listening again to my Juno-60, I'm
still convinced that something's wrong with the sound, the muddiness
and distortion with the bass, which definitely is not what I remember
from recent usage of the synth, both when using the headphones and the
mono output jack.  Since the op-amps and transistors, etc. on the jack
board are just for the headphones, I'm guessing that the damage may be
on the chorus board which provides the signal for the circuitry on the
jack board, and basically goes directly to the mono out.  Hopefully
the problem is just in the small circuit around IC8 on the chorus
board, the Toshiba TA75558S op-amp and TR20, TR22 and in the muting
ckt. the TR23 and TR21. transistors... or some caps in those ckts.

It appears that replacements for the TA75558S op-amp and transistors
may be stocked by Mouser, but I'm a little leery of their "cross
reference match" which appears mentioned in the listing of supposed
replacement products, since NTE183 (PNP) and NTE376 (NPN) are both
listed as the cross-referenced replacement for the TR20
transistor... time to check out the specs for the original parts
before ordering.

I detect a slight difference between chorus I and chorus II, with
chorus II seeming to stretch out a bit more, but not much.

Unfortunately, the op-amp, M5218L, for the jack board looks like it
might be difficult to locate, but I seem to recall that there's a
similarly numbered equivalent part available from a different
manufacturer.  Anyone know of a source for these that sells small
quantities?  I'm doubtful that will be needed since it wasn't the
headphone circuit's output that was shorted.
 
This afternoon, I opened up my Juno-60... it's been quite some time
since I've had it open.  At least there are no surface-mount parts.
First step: getting rid of the spiderwebs found in the Juno-60.  There
are these weird tiny little spiders (which grow into big nasty spiders
that bite ankles if I don't get rid of the tiny ones like them that
dangle from the rafters) here that seem to like hot areas,
particularly the area around the 200W light-bulb socket overhead---and
seem to survive, and apparently hatch, a hair's width from the
light-bulb... so, the warm insides of the Juno-60 also appeals to
them.

> like maybe an output coupling cap has gone south, or maybe a bad  
> power-on output muting transistor.  I have had more than one Juno 60  

I never used the headphone jack until this past week, and noticed the
loud popping sound when the synth is turned on and off---fortunately,
the headphones weren't on my ears... indicative of bad output muting
transistors?  Time to re-examining the schematic.  

Probably have some of those coupling caps here, but it won't hurt to
stock up on more of them.

> come across my bench with damage to their output boards due to  
> someone inadvertently connecting them to mixer inputs that had active  
> 48 Volt phantom power.  I've also seen this happen on a JX-3p.  That  

Ooohhh, nice... (sarcasm intended).  Have to watch out for that
Phantom of the Juno. ;-) Fortunately, what I connected was not powered
(it had a dead battery in it), but still possibly overloaded the
Juno-60's output.

> scenario is not too much different from the situation you  
> experienced, I think.

It looks like some similarities, but less voltage involved.  :-)  

> On each of these Junos and the JX, I discovered that both passive and  
> active components had been damaged (resistors, caps, opamps), and  
> worse yet, actual circuit traces had melted.  It was easy to fix,  

It's good to hear that was an easy fix; hopefully this will be an easy
fix as well.

> though.  The melted circuit traces were actually a good thing -- they  
> acted as fuses to prevent more of the synth from being destroyed.   

Perhaps printed circuit boards should be known as fused circuit
boards.

> Repairing the traces with jumpers and replacing a few burned parts  
> was all it took to get the synths back onto the battlefield.

Great!

> You're probably going to need a schematic and board layout diagram,  
> at least of the output board.  Do you have that?

Got them... sent away to Roland for those as soon as I bought the
synth.  Thanks for the help!

-- 
R. D. Davis 410-744-4900  Beware & halt the National Animal ID System (NAIS)! 
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